Light emitting diodes including transparent oxide layers

ABSTRACT

Light emitting diodes include a substrate having first and second opposing faces and that is transparent to optical radiation in a predetermined wavelength range and that is patterned to define, in cross-section, a plurality of pedestals that extend into the substrate from the first face towards the second face. A diode region on the second face is configured to emit light in the predetermined wavelength range, into the substrate upon application of voltage across the diode region. A mounting support on the diode region, opposite the substrate is configured to support the diode region, such that the light that is emitted from the diode region into the substrate, is emitted from the first face upon application of voltage across the diode region. A reflector is provided between the mounting support and the diode region, that is configured to reflect light that is emitted from the diode region back into the diode region, through the substrate that is transparent to optical radiation in the predetermined wavelength range and from the plurality of pedestals, upon application of voltage across the diode region. A layer of Indium Tin Oxide (ITO) is provided between the reflector and the diode region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/338,918,filed Jan. 25, 2006, entitled Light Emitting Diodes Including Pedestals,which itself is a continuation of application Ser. No. 10/859,635, filedJun. 3, 2004, entitled Light Emitting Diodes Including Pedestals, nowU.S. Pat. No. 7,026,659, which itself is a divisional of applicationSer. No. 10/057,821, filed Jan. 25, 2002, entitled Light Emitting DiodesIncluding Modifications for Light Extraction and Manufacturing MethodsTherefor, now U.S. Pat. No. 6,791,119, and claims the benefit ofProvisional Application Ser. No. 60/265,707, filed Feb. 1, 2001 entitledLight Emitting Diode With Optically Transparent Silicon CarbideSubstrate, and Provisional Application Ser. No. 60/307,235, filed Jul.23, 2001, entitled Light Emitting Diodes Including Modifications forLight Extraction and Manufacturing Methods Therefor, the disclosures ofall of which are hereby incorporated herein by reference in theirentirety as if set forth fully herein.

STATEMENT OF FEDERAL SUPPORT

This invention was made possible with government support under grantnumber 70NANB8H4022 from the National Institute of Standards andTechnology (Advanced Technology Program). The United States governmenthas certain rights to this invention.

FIELD OF THE INVENTION

This invention relates to microelectronic devices and fabricationmethods therefor, and more particularly to light emitting diodes (LEDs)and manufacturing methods therefor.

BACKGROUND OF THE INVENTION

Light emitting diodes are widely used in consumer and commercialapplications. As is well known to those having skill in the art, a lightemitting diode generally includes a diode region on a microelectronicsubstrate. The microelectronic substrate may comprise, for example,gallium arsenide, gallium phosphide, alloys thereof, silicon carbideand/or sapphire. Continued developments in LEDs have resulted in highlyefficient and mechanically robust light sources that can cover thevisible spectrum and beyond. These attributes, coupled with thepotentially long service life of solid state devices, may enable avariety of new display applications, and may place LEDs in a position tocompete with the well entrenched incandescent and fluorescent lamps.

One measure of efficiency of LEDs is the cost per lumen. The cost perlumen for an LED may be a function of the manufacturing cost per LEDchip, the internal quantum efficiency of the LED material and theability to couple or extract the generated light out of the device. Anoverview of light extraction issues may be found in the textbookentitled High Brightness Light Emitting Diodes to Stringfellow et al.,Academic Press, 1997, and particularly Chapter 2, entitled Overview ofDevice Issues in High-Brightness Light Emitting Diodes, to Craford, atpp. 47-63.

Light extraction has been accomplished in many ways, depending, forexample, on the materials that are used to fabricate the diode regionand the substrate. For example, in gallium arsenide and galliumphosphide material systems, a thick, p-type, topside window layer may beused for light extraction. The p-type window layer may be grown becausehigh epitaxial growth rates may be possible in the galliumarsenide/gallium phosphide material systems using liquid and/or vaporphase epitaxy. Moreover, current spreading may be achieved due to theconductivity of the p-type topside window layer. Chemical etching withhigh etch rates and high etch selectivity also may be used to allow theremoval of at least some of the substrate if it is optically absorbent.Distributed Bragg reflectors also have been grown between an absorbingsubstrate and the diode region to decouple the emitting and absorbingregions.

Other approaches for light extraction may involve mechanical shaping ortexturing of the diode region and/or the substrate. However, it may bedesirable to provide other light extraction techniques that can allowfurther improvements in extraction efficiency. Moreover, it may bedesirable to increase the area of an LED chip from about 0.1 mm² tolarger areas, to thereby provide larger LEDs. Unfortunately, theeffectiveness of these shaping techniques may not be maintained as thechip dimensions are scaled up for higher power/intensity and/or otherapplications.

Much development interest and commercial activity recently has focusedon LEDs that are fabricated in or on silicon carbide, because these LEDscan emit radiation in the blue/green portions of the visible spectrum.See, for example, U.S. Pat. No. 5,416,342 to Edmond et al., entitledBlue Light-Emitting Diode With High External Quantum Efficiency,assigned to the assignee of the present application, the disclosure ofwhich is hereby incorporated herein by reference in its entirety as ifset forth fully herein. There also has been much interest in LEDs thatinclude gallium nitride-based diode regions on silicon carbidesubstrates, because these devices also may emit light with highefficiency. See, for example, U.S. Pat. No. 6,177,688 to Linthicum etal., entitled Pendeoepitaxial Gallium Nitride Semiconductor Layers OnSilicon Carbide Substrates, the disclosure of which is herebyincorporated herein by reference in its entirety as if set forth fullyherein.

In such silicon carbide LEDs or gallium nitride LEDs on silicon carbide,it may be difficult to use conventional techniques for light extraction.For example, it may be difficult to use thick p-type window layersbecause of the relatively low growth rate of gallium nitride. Also,although such LEDs may benefit from the use of Bragg reflectors and/orsubstrate removal techniques, it may be difficult to fabricate areflector between the substrate and the gallium nitride diode regionand/or to etch away at least part of the silicon carbide substrate.

U.S. Pat. No. 4,966,862 to Edmond, entitled Method of Production ofLight Emitting Diodes, assigned to the assignee of the presentapplication, the disclosure of which is hereby incorporated herein byreference in its entirety as if set forth fully herein, describes amethod for preparing a plurality of light emitting diodes on a singlesubstrate of a semiconductor material. The method is used for structureswhere the substrate includes an epitaxial layer of the samesemiconductor material that in turn comprises layers of p-type andn-type material that define a p-n junction therebetween. The epitaxiallayer and the substrate are etched in a predetermined pattern to defineindividual diode precursors, and deeply enough to form mesas in theepitaxial layer that delineate the p-n junctions in each diode precursorfrom one another. The substrate is then grooved from the side of theepitaxial layer and between the mesas to a predetermined depth to defineside portions of diode precursors in the substrate while retainingenough of the substrate beneath the grooves to maintain its mechanicalstability. Ohmic contacts are added to the epitaxial layer and to thesubstrate and a layer of insulating material is formed on the diodeprecursor. The insulating layer covers the portions of the epitaxiallayer that are not covered by the ohmic contact, any portions of the onesurface of the substrate adjacent the mesas, and the side portions ofthe substrate. As a result, the junction and the side portions of thesubstrate of each diode are insulated from electrical contact other thanthrough the ohmic contacts. When the diodes are separated they can beconventionally mounted with the junction side down in a conductive epoxywithout concern that the epoxy will short circuit the resulting diode.See the abstract of U.S. Pat. No. 4,966,862.

U.S. Pat. No. 5,210,051 to Carter, Jr., entitled High Efficiency LightEmitting Diodes From Bipolar Gallium Nitride, assigned to the assigneeof the present application, the disclosure of which is herebyincorporated herein by reference in its entirety as if set forth fullyherein, describes a method of growing intrinsic, substantially undopedsingle crystal gallium nitride with a donor concentration of 7×10¹⁷ cm³or less. The method comprises introducing a source of nitrogen into areaction chamber containing a growth surface while introducing a sourceof gallium into the same reaction chamber and while directing nitrogenatoms and gallium atoms to a growth surface upon which gallium nitridewill grow. The method further comprises concurrently maintaining thegrowth surface at a temperature high enough to provide sufficientsurface mobility to the gallium and nitrogen atoms that strike thegrowth surface to reach and move into proper lattice sites, therebyestablishing good crystallinity, to establish an effective stickingcoefficient, and to thereby grow an epitaxial layer of gallium nitrideon the growth surface, but low enough for the partial pressure ofnitrogen species in the reaction chamber to approach the equilibriumvapor pressure of those nitrogen species over gallium nitride under theother ambient conditions of the chamber to thereby minimize the loss ofnitrogen from the gallium nitride and the nitrogen vacancies in theresulting epitaxial layer. See the abstract of U.S. Pat. No. 5,210,051.

In view of the above discussion, improved light extraction techniquesmay be desirable for LEDs, especially LEDs that are fabricated fromsilicon carbide, that are fabricated from gallium nitride on siliconcarbide and/or that have a relatively large area.

SUMMARY OF THE INVENTION

Light emitting diodes according to some embodiments of the inventioninclude a substrate having first and second opposing faces that istransparent to optical radiation in a predetermined wavelength range andthat is patterned to define, in cross-section, a plurality of pedestalsthat extend into the substrate from the first face towards the secondface. As used herein, the term “transparent” refers to an element, suchas a substrate, layer or region that allows some or all opticalradiation in a predetermined wavelength range to pass therethrough,i.e., not opaque. A diode region on the second face is configured toemit light in the predetermined wavelength range, into the substrateupon application of voltage across the diode region. In otherembodiments, a mounting support on the diode region, opposite thesubstrate is configured to support the diode region, such that the lightthat is emitted from the diode region into the substrate, is emittedfrom the first face upon application of voltage across the diode region.In some embodiments, the light emitting diode on a transparent substratewith pedestals is flip-mounted on a mounting support, with the dioderegion adjacent to the mounting support and a substrate opposite themounting support, for light emission through the substrate. In otherembodiments, the light emitting diode on a transparent substrate withpedestals is mounted on a mounting support, with the substrate adjacentto the mounting support and the diode region opposite the mountingsupport. Thus, non-flip-chip mounting also may be provided.

In yet other embodiments of the invention, a reflector also is providedbetween the mounting support and the diode region or the substrate. Thereflector may be configured to reflect light that is emitted from thediode region back through the diode region, through the substrate andfrom the pedestals, upon application of voltage across the diode region.In other embodiments, a transparent electrode also may be providedbetween the diode region and the reflector. In still other embodiments,a solder preform and/or other bonding region may be provided between thereflector and the mounting support and/or an optical element such as awindow or lens may be provided adjacent the first face opposite thediode region. In yet other embodiments, the diode region includes aperipheral portion and at least one central portion that is enclosed bythe peripheral portion, and the light emitting diode further comprisesat least one electrode on the diode region, that is confined to withinthe at least one central portion and does not extend onto the peripheralportion. It will be understood that the central portion need not becentered on the diode region.

In other embodiments of the invention, a contact structure for thesubstrate and/or the diode region of an LED includes a transparent ohmicregion, a reflector, a barrier region and a bonding region. Thetransparent ohmic region provides electrical contact and/or currentspreading. The reflector reflects at least some incident radiation andalso may provide current spreading. The barrier region protects thereflector and/or the ohmic region. The bonding region bonds the LEDpackage to a mounting support. In some embodiments, the functionality ofthe transparent ohmic region and the reflector can be combined in asingle ohmic and reflector region. Contact structures according to theseembodiments of the invention also may be used with conventional siliconcarbide LEDs, gallium nitride on silicon carbide LEDs and/or other LEDs.

In still other embodiments of the present invention, the first face ofthe substrate may include therein at least one groove that defines aplurality of pedestals, such as triangular pedestals, in the substrate.The grooves may include tapered sidewalls and/or a beveled floor. Thefirst and second faces of the substrate may have square perimeters,and/or the first face of the substrate may be textured. The lightemitting diode may further include a plurality of emission regionsand/or electrodes on the diode region, a respective one of which isconfined to within a respective one of the pedestals and does not extendbeyond the respective one of the pedestals.

In yet other embodiments of the present invention, the first face of thesubstrate includes therein an array of via holes. The via holes mayinclude tapered sidewalls and/or a floor. The via holes preferablyextend only part way through the substrate, but in other embodimentsthey can extend all the way through the substrate. The first and secondsubstrate faces may have square perimeters, and/or the first face may betextured. The light emitting diodes may further include at least oneelectrode on the diode region that does not overlap the array of viaholes.

The pedestals and/or array of via holes also may be used with lightemitting diodes that include silicon carbide or non-silicon carbidesubstrates, to allow improved light extraction therefrom. Moreover,electrodes as described above also may be used with light emittingdiodes that include a non-silicon carbide substrate. For example, whenthe first face of the substrate has smaller surface area than the secondface, and the diode region is on the second face, an emission region maybe provided on the diode region that is confined to within the smallersurface area of the first face.

In other embodiments of the present invention, light emitting diodesinclude a compensated, colorless silicon carbide substrate having firstand second opposing faces and a gallium nitride-based diode region onthe second face that is configured to emit light into the substrate uponapplication of voltage across the diode region. Mounting supports,reflectors, contact structures, grooves, pedestals, texturing and/orconfined emission areas/electrodes may be provided according to any ofthe embodiments that were described above.

Accordingly, many of the above-described embodiments compriseembodiments of means for extracting from the substrate at least some ofthe light that is emitted into the substrate by the diode region.Examples of these means for extracting include compensating dopants inthe silicon carbide substrate to provide a colorless silicon carbidesubstrate, patterning the substrate to define, in cross-section, aplurality of pedestals that extend into the substrate from the firstface toward the second face and/or many of the other embodiments thatwere described above, including mounting supports, reflectors, contactstructures, grooves, pedestals, texturing and/or confined emissionareas/electrodes.

Light emitting diodes may be manufactured, according to some embodimentsof the invention, by forming a diode region that is configured to emitlight in a predetermined wavelength range on a second face of asubstrate having first and second opposing faces, and that istransparent to the optical radiation in the predetermined wavelengthrange. The substrate is patterned before, during and/or after formingthe diode region to define, in cross-section, a plurality of pedestalsthat extend into the substrate from the first face towards the secondface. In other embodiments, the diode region is mounted onto a mountingsubstrate that is configured to support the diode region such that thelight that is emitted from the diode region into the substrate isemitted from the first face upon application of voltage across the dioderegion. The mounting may be preceded by forming a reflector on the dioderegion such that the reflector is configured to reflect light that isemitted from the diode region back into the diode region through thesubstrate and from the first face, upon application of voltage acrossthe diode region. Prior to forming the reflector, a transparent ohmicelectrode also may be formed on the diode region opposite the substrate.A barrier region and/or an adhesion region also may be formed afterforming the reflector. In other embodiments, a mounting support isplaced adjacent the reflector with the barrier region and/or theadhesion region therebetween, and the LED is joined to the mountingsupport. In still other embodiments, an electrode is formed on the dioderegion that is confined to within the central portion thereof and doesnot extend onto the peripheral portion thereof.

Other method embodiments include forming a plurality of intersectinggrooves into the first face of the substrate to define the plurality ofpedestals, such as triangular pedestals, in the substrate. The groovesmay include tapered sidewalls and/or a beveled floor. The first face ofthe substrate also may be textured. A plurality of electrodes also maybe formed on the diode region. In some embodiments, a respective one ofthe electrodes is confined to within a respective one of the pedestalsand does not extend beyond the respective one of the pedestals.

Still other method embodiments according to the present inventioninclude reactive ion etching an array of via holes in the first face ofthe substrate. The via holes may include tapered sidewalls and/or afloor. The first face also may be textured. An electrode may be formedon the diode region that does not overlap the array of via holes.

Sawing a plurality of intersecting grooves and/or reactive etching anarray of via holes into the first face may be used for light emittingdiodes that include a silicon carbide or non-silicon carbide substrateto allow improved light extraction therefrom. Moreover, the formation ofan emission region on the diode region that is confined to within thesmaller surface area of the first face also may be used for otherconventional light emitting diodes, to allow improved light extractiontherefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 are cross-sectional views of light emitting diodes accordingto embodiments of the present invention.

FIG. 6 graphically illustrates absorption of light versus wavelength forsilicon carbide at various doping levels.

FIG. 7A is a top view and FIGS. 7B and 7C are cross-sectional viewsalong the line 7B-7B′ of FIG. 7A, of light emitting diodes according toother embodiments of the present invention.

FIG. 8A is a top view and FIGS. 8B and 8C are cross-sectional viewsalong the line 8B-8B′ of FIG. 8A, of light emitting diodes according toother embodiments of the present invention.

FIGS. 9-13 are cross-sectional views of light emitting diodes accordingto yet other embodiments of the present invention.

FIG. 14A a cross-sectional view taken along the lines 14A-14A′ of FIG.14B, which is a bottom view of light emitting diodes according to stillother embodiments of the present invention.

FIG. 15A a cross-sectional view taken along the lines 15A-15A′ of FIG.15B, which is a bottom view of light emitting diodes according to yetother embodiments of the present invention.

FIGS. 16, 17A and 18 are cross-sectional views of light emitting diodesaccording to still other embodiments of the present invention.

FIG. 17B is a top view of FIG. 17A according to embodiments of thepresent invention.

FIG. 19 is a flowchart illustrating manufacturing methods for lightemitting diodes according to embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. Like numbers refer to like elements throughout. It will beunderstood that when an element such as a layer, region or substrate isreferred to as being “on” or extending “onto” another element, it can bedirectly on or extend directly onto the other element or interveningelements may also be present. In contrast, when an element is referredto as being “directly on” or extending “directly onto” another element,there are no intervening elements present. Moreover, each embodimentdescribed and illustrated herein includes its complementary conductivitytype embodiment as well.

Embodiments of the invention now will be described, generally withreference to gallium nitride-based light emitting diodes on siliconcarbide-based substrates. However, it will be understood by those havingskill in the art that many embodiments of the invention may be employedwith any combination of a substrate that is non-absorbing or transparentto the emitted light and an index matched light emitting diode epitaxiallayer. In some embodiments, the refractive index of the substrate isgreater than that of the diode. Accordingly, combinations can include anAlGaInP diode on a GaP substrate; an InGaAs diode on a GaAs substrate;an AlGaAs diode on a GaAs substrate; an SiC diode on an SiC Substrate,an SiC diode on a sapphire (Al₂O₃) substrate; and/or a nitride-baseddiode on a gallium nitride, silicon carbide, aluminum nitride, zincoxide and/or other substrate.

FIG. 1 is a cross-sectional view of light emitting diodes according tosome embodiments of the present invention. As shown in FIG. 1, theselight emitting diodes 100 include a silicon carbide substrate 110 havingfirst and second opposing faces 110 a and 110 b and that is transparentto optical radiation in a predetermined wavelength range. A diode region170 is on the second face 110 b and is configured to emit light in thepredetermined wavelength range into the silicon carbide substrate 110that is transparent to optical radiation in the predetermined wavelengthrange, upon application of voltage across the diode region, for exampleacross ohmic contacts 150 and 160.

Still referring to FIG. 1, the diode region 170 includes an n-type layer120, an active region 130, and a p-type layer 140. Ohmic contacts 150and 160 are made to the p-type layer 140 and the n-type layer 120,respectively, to form an anode and a cathode, respectively. The dioderegion 170 including the n-type layer 120, the active region 130, and/orthe p-type layer 140 preferably comprise gallium nitride-basedsemiconductor layers, including alloys thereof such as indium galliumnitride and/or aluminum indium gallium nitride. The fabrication ofgallium nitride on silicon carbide is known to those having skill in theart, and is described for example in the above-incorporated U.S. Pat.No. 6,177,688. It also will be understood that a buffer layer or layers,for example comprising aluminum nitride, may be provided between then-type gallium nitride layer 120 and the silicon carbide substrate 110,for example as described in U.S. Pat. Nos. 5,393,993, 5,523,589,6,177,688, and application Ser. No. 09/154,363 entitled VerticalGeometry InGaN Light Emitting Diode, the disclosures of which are herebyincorporated herein by reference in their entirety as if set forth fullyherein.

The active region 130 may comprise a single layer of n-type, p-type orintrinsic gallium nitride-based materials, another homostructure, asingle heterostructure, a double heterostructure and/or a quantum wellstructure, all of which are well known to those having skill in the art.Moreover, the active region 130 may comprise a light emitting layerbounded by one or more cladding layers. Preferably, the n-type galliumnitride layer 120 comprises silicon-doped gallium nitride, while thep-type gallium nitride layer 130 comprises magnesium-doped galliumnitride. In addition, the active region 130 preferably includes at leastone indium gallium nitride quantum well.

In some embodiments, the ohmic contact 150 for the p-type galliumnitride layer 140 comprises platinum, nickel and/or titanium/gold. Inother embodiments, a reflective ohmic contact comprising, for example,aluminum and/or silver, may be used. The ohmic contact 160 to the n-typegallium nitride 120 preferably comprises aluminum and/or titanium. Othersuitable materials that form ohmic contacts to p-type gallium nitrideand n-type gallium nitride may be used for ohmic contacts 150 and 160,respectively. Examples of ohmic contacts to n-type gallium nitride andp-type gallium nitride are shown, for example in U.S. Pat. No.5,767,581, the disclosure of which is hereby incorporated herein byreference in its entirety as if set forth fully herein.

Still referring to FIG. 1, in some embodiments, the substrate 110comprises a silicon carbide substrate that is transparent to opticalradiation in the predetermined wavelength range. One technique forfabricating a silicon carbide substrate that is transparent to opticalradiation in a predetermined wavelength range is described in U.S. Pat.No. 5,718,760, which is assigned to the assignee of the presentinvention, the disclosure of which is hereby incorporated herein in itsentirety as if set forth fully herein. Silicon carbide substrate 110 maycomprise the 2H, 4H, 6H, 8H, 15R and/or 3C polytypes. The 6H and/or 4Hpolytype may be preferred for optoelectronic applications.

In other embodiments, the silicon carbide substrate 110 is acompensated, colorless silicon carbide substrate, as described in theabove-cited U.S. Pat. No. 5,718,760. As described therein, colorlesssilicon carbide may be fabricated by sublimation of silicon carbide inthe presence of compensating amounts of p-type and n-type dopants.Naturally-occurring silicon carbide typically is black, due to highimpurity levels. Conventional microelectronic silicon carbide wafershave a translucent blue, amber or green hue depending upon thecontrolled doping level in the crystal. As described in U.S. Pat. No.5,718,760, it was found that by carefully controlling the doping ofsilicon carbide crystals with compensating levels of n-type and p-typedopants at low doping concentrations, colorless single crystals ofsilicon carbide may be obtained. In particular, it may be desirable toreduce and preferably minimize the unintentional nitrogen (n-type)doping in the material and to introduce low levels of compensatingp-type dopants, to thereby create colorless silicon carbide.

As seen in FIG. 6, 4H—SiC is characterized by an absorption peak ataround 460 nm. FIG. 6 shows that by reducing doping levels in 4H—SiC,that absorption peak can be substantially reduced, so that the 4H—SiCbecomes transparent at around 460 nm. The curve labeled 4H-HD showsmeasured absorption values for 4H silicon carbide doped with a net donorconcentration of approximately 2.5×10¹⁸, while the curve labeled 4H-LDshows measured absorption values for 4H silicon carbide doped with a netdonor concentration of approximately 5×10¹⁷. Further reduction in dopinglevels may result in even lower absorption levels. The curve labeled 6Halso shows a transparent substrate at around 460 nm.

In accordance with some embodiments of the present invention, colorlessboules of silicon carbide grown, for example, according to processesdescribed in U.S. Pat. No. 5,718,760 and references cited therein, maybe cut into wafers for processing. Gallium nitride-based epitaxiallayers may be formed on the wafers, for example, as described in U.S.Pat. No. 6,177,688, which then can be processed to produce structuressuch as are shown in FIG. 1.

In LEDs having silicon carbide substrates, it previously may have beenpreferable to prevent light generated in the active region from enteringthe substrate, for a number of reasons. For example, although siliconcarbide may be highly transparent compared to gallium arsenide,conventional silicon carbide substrates can absorb some light inportions of the visible spectrum. Moreover, since conventional siliconcarbide devices are vertical devices that are mounted with thesubstrates facing down, some light entering the substrate can bereflected back through the substrate before it is extracted from thedevice, thereby increasing absorption losses in the substrate.Reflection losses also may reduce the overall efficiency of the device.

Gallium nitride and silicon carbide have similar indices of refraction.In particular, gallium nitride has an index of refraction of about 2.5,while silicon carbide has an index of refraction of about 2.6-2.7. Inthat sense, gallium nitride and silicon carbide may be said to beoptically matched. Thus, very little internal reflection may occur at aboundary between gallium nitride and silicon carbide. Consequently, itmay be difficult to prevent light generated in a gallium nitride-basedlayer from passing into a silicon carbide substrate.

By providing a compensated, colorless silicon carbide substrate that isgrown, for example, in accordance with methods described in U.S. Pat.No. 5,718,760, absorption of visible light in the silicon carbidesubstrate may be reduced. As is well known to one skilled in the art,absorption losses may increase as doping increases due to the so-called“Biedermann Effect”. Consequently, extraction of visible light from thesubstrate may be enhanced when doping is reduced and preferablyminimized, thereby improving the overall efficiency of the device. Incontrast to silicon carbide, sapphire has an index of refraction ofabout 1.8. Thus, in a sapphire-based gallium nitride LED, a largeportion of light generated in the gallium nitride active layer may notpass into the substrate but will be reflected away from the substrate.

In other embodiments of the present invention, the doping of the siliconcarbide substrate may be controlled such that light in the wavelengthrange generated in the diode region 170 of the device is not absorbed bythe substrate 110, although other wavelengths may be absorbed. Thus, theabsorption characteristics of the silicon carbide substrate may beadjusted to pass desired wavelengths of light. For example, the activeregion 130 may be designed to emit blue light in the vicinity of 450 nm.The doping of the silicon carbide substrate 110 may be controlled suchthat light rays having a wavelength of approximately 450 nm are notsubstantially absorbed by the substrate 110. Thus, although thesubstrate may not be entirely colorless, and other wavelengths may beabsorbed, it nevertheless may be transparent to the wavelengths ofinterest, namely those generated in the LED region 170. In preferredembodiments, the bandgap and/or doping of the silicon carbide substrate110 is controlled such that the substrate is transparent to light withinthe range of about 390-550 nm.

Thus, in some embodiments, the substrate 110 may be thought of as afilter which may improve the spectral purity of light output by thedevice. For example, it is known to those skilled in the art thatgallium nitride-based blue LEDs may produce unwanted emission in theultraviolet (UV) spectrum in addition to the desired emission. Such UVemissions may be undesirable at even moderately low power levels sincethey may degrade the plastic materials in which LEDs are packaged, whichmay result in reliability problems and/or reduced lifetimes. It is alsoknown that 6H silicon carbide absorbs UV light. Thus, it may bepreferable to extract light through a 6H silicon carbide substrate,which filters out unwanted UV emissions.

Instead of discouraging or inhibiting light from entering the substrate,as may be done conventionally, embodiments of the present invention canencourage light generated in the diode region 170 to enter the substrate110, where it can be most efficiently extracted. Thus, some embodimentsof the invention can provide means for extracting from the substrate, atleast some of the light that is emitted into the substrate by the dioderegion. Accordingly, some embodiments of the present invention may beparticularly suited for use in a so-called “flip-chip” or “upside-down”packaging configuration as will now be described in connection with FIG.2. Embodiments of the invention also may be used with conventional“right-side-up” or “non-flip-chip” packaging, as will be described inconnection with FIG. 16.

Referring now to FIG. 2, other embodiments of light emitting diodesaccording to embodiments of the present invention are shown. In FIG. 2,a light emitting diode 200 is shown flip-chip, or upside-down mounted ona mounting support 210, such as a heat sink, using bonding regions 220and 230. The bonding regions 220 and 230 may include solder preformsthat are attached to the diode region and/or the mounting support 210,and that can be reflowed to attach the ohmic contacts 150 and 160 to themounting support 210 using conventional solder reflowing techniques.Other bonding regions 220 and 230 may comprise gold, indium and/orbraze. As also shown in FIG. 2, the flip-chip or upside-down packingconfiguration places the silicon carbide substrate 110 up, away from themounting substrate 210, and places the diode region 170 down, adjacentto the mounting substrate 210. A barrier region (not shown) also may beincluded between a respective ohmic contact 150, 160 and a respectivemelting region 220, 230. The barrier region may comprise nickel,nickel/vanadium and/or titanium/tungsten. Other barrier regions also maybe used.

Still referring to FIG. 2, as shown by light ray 250, light generated inthe active region 130 enters the substrate 110, and exits the devicefrom the first face 110 a of the substrate 110. In order to encouragelight generated in the active region 130 to enter the substrate 110, areflector 240 may be provided that is positioned between the activeregion 130 and the mounting support 210, opposite the substrate 110. Thereflector 240 may be positioned between the active region 130 and thep-type layer 140 as shown in FIG. 2. However, there may be one or moreintervening layers between the reflector 240 and the active region 130and/or the p-type layer 140. Moreover, the p-type layer 140 may bepositioned between the reflector 240 and the active region 130. Otherconfigurations also may be provided, for example, as will be describedin connection with FIG. 16 below.

The reflector 240 may comprise an aluminum gallium nitride (AlGaN) layerand/or a distributed Bragg reflector that can reflect light from theactive region 130 back towards the substrate 110. The design andfabrication of Bragg reflectors is well known to those having skill inthe art and is described, for example, in U.S. Pat. No. 6,045,465, thedisclosure of which is hereby incorporated herein by reference in itsentirety as if set forth fully herein. It also will be understood thatthe reflector also can modify the pattern of photon emission from theactive region 130 itself, thereby directing more photons to escape thedevice. Other conventional reflector structures also may be used.

Still referring to FIG. 2, the exemplary ray 250 is shown to illustratehow the light that is generated within the active region 130 mayinitially travel in a direction away from the substrate 110, but will bereflected by the reflector 240 back through the substrate 110 and out ofthe device 200. It will be noted that in the flip-chip configurationthat is illustrated in FIG. 2, the ray 250 only needs to travel throughthe substrate 110 one time before exiting the device.

LEDs 200 according to embodiments of the invention may be packaged inconventional dome structures 280 that include an optical element such asa lens 282 for light emission. The entire dome 280 may function as anoptical element. An anode lead 260 and a cathode lead 270 also may beprovided for external connections. The dome structure 280 may compriseplastic, glass and/or other materials and also may include silicone geland/or other materials therein.

Referring now to FIG. 3, vertical light emitting diodes 300 also may bepackaged in a flip-chip configuration by providing a cathode ohmiccontact 160′ on the first face 110 a of the silicon carbide substrate110, and providing a wire 390 or other electrical connection between thecathode ohmic contact 160′ on the first face 110 a to the externalcathode lead 270. Non-flip-chip configurations also may be provided, forexample, as will be described in connection with FIG. 16 below.

FIG. 4 illustrates other LEDs according to embodiments of the invention.In these LEDs 400, an anode contact may include an ohmic and reflectiveregion 410, which may include a plurality of layers, including a thintransparent ohmic contact 412 and a reflector 414. The thin transparentohmic contact 412 may comprise platinum, and preferably should be asthin as possible to avoid substantial absorption of light. The thicknessof the thin transparent ohmic contact 412 preferably is between about 10Å and about 100 Å for a platinum transparent electrode 412. In otherembodiments, the thin transparent ohmic contact 412 may comprisenickel/gold, nickel oxide/gold, nickel oxide/platinum, titanium and/ortitanium/gold, having thickness between about 10 Å and about 100 Å. Thereflector 414 preferably is thicker than about 300 Å and preferablycomprises aluminum and/or silver. Embodiments of FIG. 4 can provideimproved current spreading, since the reflector 414 contacts the thintransparent ohmic contact 412 over the entire surface area of the thintransparent ohmic contact 412. Thus, current need not travelhorizontally through the anode contact 410, as may be the case inconventional devices. Current spreading thus may be enhanced. Devices400 of FIG. 4 also may be packaged as was shown in FIGS. 2 and 3. Othercontact structures may be used, for example, as will be described indetail below in connection with FIGS. 16 and 17. For example, a barrierregion 155 comprising, for example, nickel, nickel/vanadium and/ortitanium/tungsten may be provided between the reflector 414 and thebonding region 220 and between the ohmic contact 160 and the bondingregion 230.

LEDs according to other embodiments of the present invention areillustrated in FIG. 5. These embodiments of LEDs 500 can enhance lightextraction from the LED by beveling or slanting at least some of thesidewall 110 c of the substrate 110′. Since the incident angle of lightstriking the beveled sidewall 110 c is generally closer to the normalthan it otherwise might be, less light may be reflected back into thesubstrate 110′. Accordingly, light may be extracted from the substrate110′ which can improve the overall efficiency of the device. Extractionefficiency may be further improved by roughening or texturing thesidewalls 110 c and/or the first face 110 a, of the substrate 110′, forexample using conventional methods.

Accordingly, light emitting diodes according to some embodiments of theinvention include a substrate and a gallium nitride-based diode region.The substrate comprises single crystal, transparent silicon carbide foran emission range of interest, preferably fabricated via sublimation.The substrate may be between about 100 μm and about 1000 μm thick.External efficiency of the diode can be enhanced due to increased lightextraction from the substrate. In some embodiments of the invention, thediode includes a reflector that reflects light generated in the dioderegion back into the substrate for subsequent extraction from thedevice. The reflector may comprise a layer of material with a relativelylow index of refraction (such as AlGaN) on the active region oppositethe substrate. Alternatively, the reflector may comprise a Braggreflector within the structure and/or a coating layer of aluminum and/orsilver on a transparent ohmic contact. Other embodiments will bedescribed below. In yet other embodiments, a portion of the sidewalls ofthe substrate may be tapered and/or roughened to allow improved lightextraction. Diodes according to embodiments of the present invention maybe particularly suited for use in a flip-chip mounting structure.However, non-flip-chip mounting also may be used.

Embodiments of the invention that were described in FIGS. 1-6 aboveprovide modifications of the silicon carbide substrate, to embody meansfor extracting at least some of the light in a predetermined wavelengthrange, and thereby allow flip-chip or non-flip-chip mounting of galliumnitride on silicon carbide LEDs. Other embodiments of the invention nowwill be described where various geometric modifications are made to thesubstrate, to provide other embodiments of means for extracting at leastsome of the light from the substrate, to allow increased extractionefficiency. These substrate modifications may be especially useful whenlarge area chips are being fabricated, to provide other embodiments ofmeans for extracting, from the substrate, at least some of the light,and allow enhanced extraction efficiency from interior regions of thesubstrate. These enhancements may be used with silicon carbidesubstrates, as was described in connection with FIGS. 1-6 above, butalso may be used with conventional substrates comprising galliumarsenide, gallium phosphide, alloys thereof and/or sapphire.

FIG. 7A is a top view of LEDs according to other embodiments of thepresent invention. FIGS. 7B and 7C are cross-sectional views of LEDs ofFIG. 7A taken along line 7B-7B′ of 7A, according to other embodiments ofthe present invention.

Referring now to FIGS. 7A-7C, these LEDs include a substrate 710 havingfirst and second opposing faces 710 a and 710 b, respectively, and adiode region 740 on the second face 710 b of the substrate 710. Thesubstrate 710 may be a silicon carbide substrate 110, such as wasdescribed in FIGS. 1-6, and/or another conventional LED substrate. Thediode region 740 may include a diode region 170 of FIGS. 1-5 above,and/or any other conventional diode region.

As also shown in FIG. 7A, the first face 710 a of these LEDs 700 includetherein a plurality of grooves 720 that define a plurality of pedestals730 in the substrate. In FIG. 7A, triangular pedestals are shown.However, pedestals of other polygonal or non-polygonal shapes may beprovided. As shown in FIG. 7B, the grooves 720 may include a beveledfloor 722. However, a flat floor also may be included. Moreover,although the sidewalls 724 of the grooves are shown as being orthogonalto the first and second faces 710 a and 710 b, tapered sidewalls alsomay be included, wherein the cross-sectional area of the sidewallpreferably decreases from the first face 710 a to the second face 710 bof the substrate 710.

FIG. 7C illustrates other embodiments where nonplanar features areprovided. Thus, a wide saw cut or other technique can be used to formpedestals 730′ that have tapered and curved sidewalls 724, curved floors722′ and/or domed first faces 710 a′ in the pedestals 710′. Theseembodiments of pedestals 730′ can thereby form a lens-like structurethat can reduce total internal reflection. Alternatively, rather than acurved or lens-shaped first face 710 a′ and/or floor 722′, facets may beformed on the first face 710 a′ and/or the floor 722′, to furtherenhance light extraction. Etching, saw cutting, laser cutting and/orother conventional techniques may be used to produce these structures.

Finally, as also shown in FIGS. 7A-7C, the perimeters of the first andsecond faces 710 a, 710 a′ and 710 b are square. However, it will beunderstood that other shapes may be used. For example, triangularperimeter-shaped first and second faces may be used. Moreover, althoughfour triangular pedestals 730 are shown, two or more pedestals may beused, and preferably more than four pedestals are used in relativelylarge chips having an area, for example, of greater than 0.1 mm².

FIG. 8A is a top view of LEDs according to yet other embodiments of thepresent invention. FIGS. 8B and 8C are cross-sectional views of LEDs ofFIG. 8A taken along line 8B-8B′ of 8A, according to yet otherembodiments of the present invention.

As shown in FIG. 8A, these light emitting diodes 800 include a substrate810 having first and second opposing faces 810 a and 810 b, and a dioderegion 840 on the second face 810 b. The substrate 810 may be a siliconcarbide substrate, such as the silicon carbide substrate 110 that wasdescribed in connection with FIGS. 1-6, and/or any other conventionalLED substrate. The diode region 840 can be a gallium nitride-based dioderegion 170, such as was described in FIGS. 1-5, and/or any otherconventional diode region.

As shown in FIGS. 8A and 8B, the substrate 810 includes an array of viaholes 820 in the first face 810 a. The via holes 820 preferably extendonly part way through the substrate 810, but in other embodiments, theycan extend all the way through the substrate 810. As also shown, the viaholes can include tapered sidewalls 824. The sidewalls 824 can be curvedor straight. Moreover, curved or straight sidewalls that are orthogonalto the first and second faces 810 a and 810 b also may be used. The viaholes 820 may have a flat, beveled and/or curved floor 822, to providefrusto-conical or cylindrical via holes. The via holes also may notinclude a floor 822, but rather may come to a point, to provide conicalvia holes. Although an array of four complete via holes and twelvepartial via holes are shown in FIGS. 8A and 8B, two or more via holes820 may be used, and more than four full via holes 820 preferably areincluded for large area chips, for example chips having an area greaterthan 0.1 mm².

As shown in FIG. 8B, the first face can be a beveled first face 810 athat includes one or more facets therein. In other embodiments, the tops810 a may be rounded to provide a lens-like structure.

As will be described in detail below, the grooves 720 of FIGS. 7A-7C maybe fabricated using a dicing saw, for example at a 45° angle to the chipsides, prior to or after dicing the chip. Other techniques includingreactive ion etching through a mask, laser cutting, wet etching and/orother techniques may be used. The via holes 820 of FIGS. 8A-8C may befabricated using reactive ion etching through a mask, prior to or afterdicing the chip.

Light extraction through LEDs of FIGS. 7A-7C and 8A-8C now will bedescribed. The grooves 720 or via holes 820 can allow light extractionto occur periodically across the chip, rather than just along the edgesof the chip. They can, therefore, provide scalability for large areachips. This contrasts sharply from dicing grooves that are placed in asubstrate to permit dicing the substrate in the grooves and alsocontrasts sharply from grooves that are cut through the diode region ofan LED.

The pedestals, via holes, sidewalls and/or grooves can improve lightextraction because the vertical edges of the chip generally play a rolein light extraction. Furthermore, for purposes of light extraction, thebest shape for an LED chip may not be a square or rectangular perimeter.However, LED chips generally have square perimeters for reasons ofpacking density within a wafer. In a square chip, a ray striking asidewall from any direction at an angle greater than the critical angle,and less than the complement of this angle, generally will be lost tointernal reflections and subsequent absorption. Cylindrical chips mayreduce internal reflections, but their manufacturability and packingdensity may be poor. Moreover, light generated at points further fromthe center of a cylindrical die may increasingly produce rays thatstrike the vertical sidewalls tangentially. More rays are once againlost to internal reflections and absorption. The overall die area thenmay need to be large compared to the active central area, which may bean inefficient use of wafer area, and may lead to higher cost.

In sharp contrast, some embodiments of the present invention can formpedestals, such as equilateral or non-equilateral triangular pedestals730, in the substrate. Excellent wafer utilization may be maintained. Inparticular, a generated ray may have no more than one incidence andreflection at a sidewall 724, at an angle greater than the criticalangle. Angles of incidence greater than the critical angle arereflected, but they may strike the next wall at less than the criticalangle in all cases, assuming that the index of refraction for theencapsulating material is about 1.5 or greater. Thus, unlike die withsides at right angles or smooth continuous arcs, no ray may be lost inits entirety to internal reflection and absorption.

LEDs having a triangular shape versus a square LED with the same chiparea, running at identical currents and having smooth vertical sidewallscan, for example, yield a 15% improvement in light output. In addition,tapered sidewalls or channels can be used with the triangular pedestals,to access even more of the trapped light in the substrate. Finally,triangular pedestals that may be formed using grooves cut at 45° anglesor other angles relative to the square die can allow for standard diehandling and separation techniques, but can provide for the extra lightextraction advantage of a non-square-shaped chip. Standard edge shapingand die separation may be used. Similar effects may be provided using anarray of via holes 820.

It will be understood that the LEDs of FIGS. 7A-7C and 8A-8C may bemounted upside down or in flip-chip configuration, as was illustrated inFIGS. 2-5. Silver epoxy that may be used to couple the LED to a mountingsubstrate may be prevented from entering the grooves or via holes, whichcould reduce the efficiency thereof. In other embodiments, if thegrooves and/or via holes are used with conventional, non-flip-chip LEDmounting, a reflective back layer, such as silver or aluminum, can beformed on the first face of the substrate, so that light incident on thegrooves or via holes will be reflected back towards and through thediode region.

Additional discussion of the use of solder preforms in the bondingregions 220/230 now will be provided. Small area LEDs may requireexceptional care to prevent the silver epoxy that is used in attachingthe die to the lead frame from contacting the sidewall of the chipand/or the conducting substrate. Such a contact may form a Schottkydiode, which may be detrimental to the performance of vertical LEDstructures. The Schottky diode can shunt current around the LED becauseit has a lower forward turn-on voltage. The use of silver epoxy on alarge chip may be more manageable than on a small chip, sinceover-dispensing may not cause the epoxy to come out from under the chipand possibly reach the vertical sidewall of the diode region and/orsubstrate. According to embodiments of the invention, a solder preformmay be formed on or attached to the reflector or other layer, as will bedescribed below. The preform may comprise a low temperature eutecticalloy, such as lead-tin, indium-gold, gold-tin and/or silver-tin solder.The preform shape can be well defined and the outward creep can becontrolled by the pressure and/or temperature employed in the die attachprocess. Moreover, the thermal conductivity of a preform may be superiorto silver epoxy, which can be advantageous for high-power devices.

Referring now to FIGS. 9 and 10, texturing of the substrate also may beprovided. The texturing may be on the order of an emission wavelength orlarger. For example, as shown in FIG. 9, LEDs 900 may include a texturedfirst face 710 a′ of the substrate 710. Textured sidewalls 724′ and/ortextured floors 722′ may be provided in addition to or instead of thetextured first face 710 a′. As shown in FIG. 10, LEDs 1000 may include atextured first face 810 a′ that can include an array of microlenses 1010thereon. Textured sidewalls and/or floors also may be provided. Anyconventional technique for texturing may be used. Texturing, asdescribed for example in connection with FIGS. 9-10, also may be usedwith other embodiments that are described herein.

It may be preferable for the exposed surface of the packaged LED chip tobe textured and not polished. Conventional chips are packaged with apolished epitaxial side up, which can reduce light extraction therefrom.Texturing the exposed surface can provide a random probability thatincident light can be transmitted instead of being internally reflected.It also will be understood that texturing the substrate backside neednot preclude the formation of an ohmic contact to this substrate face.More particularly, vertical LEDs may have one contact to the activediode region and a backside contact to the backside of the substratethat is textured. Texturing of the sidewalls can provide up to 20% ormore light emission compared to a polished surface.

FIGS. 11 and 12 illustrate other LEDs according to embodiments of thepresent invention. LEDs 1100 of FIG. 11 and LEDs 1200 of FIG. 12 maycorrespond to the LEDs 700 of FIG. 7 and LEDs 800 of FIG. 8. However, inthe embodiments of FIGS. 11 and 12, a transparent ohmic contact 412 anda reflector 414 are added. The thin transparent ohmic contact 412 canimprove the current spreading of the p-type gallium nitride layer andcan make ohmic contact to the anode, while preferably blocking areduced, and more preferably a minimal, amount of light.

Moreover, in the configurations shown in FIGS. 11 and 12, since the chipis flip-chip mounted on the mounting substrate 210, the transparentohmic contact 412 can be thinned more than may be possible in aconventional nitride LED, thus allowing it to be more transparent. Arelatively thick reflector 414, comprising, for example, aluminum and/orsilver, also can be provided. The reflector 414 can provide excellentcurrent spreading. Moreover, a solder preform 220, and/or other mountingregion may be used between the metal contact 155 and the mountingsubstrate 210, to provide electrical and mechanical connection and heattransfer from the diode region, while preventing a short circuit in thediode region, which could create a parasitic Schottky contact. It willbe understood that by flip-chip mounting the diode, power dissipationneed not occur through the substrate. Rather, the heat generating dioderegion can be in intimate contact with the heat sink, with a lowerthermal resistance. In other embodiments, described, for example, inconnection with FIG. 16, non-flip-chip mounting may be used. It alsowill be understood that a transparent/reflector electrode, solderpreform and/or flip-chip mounting may be used with other embodiments ofthe invention described herein.

FIG. 13 illustrates other embodiments of the invention that can be usedto improve extraction efficiency in a conventional ATON LED. As is wellknown to those having skill in the art, an ATON LED uses substrateshaping as described, for example, in the publication entitled OSRAMEnhances Brightness of Blue InGaN LEDs, Compound Semiconductor, Volume7, No. 1, February 2001, p. 7. In particular, as shown in FIG. 13, aconventional ATON LED 1300 includes a substrate 1310 and a diode region1320. According to embodiments of the invention, an emission region,defined, for example, by a mesa 1320 a and/or an electrode 1330, isincluded only in a central portion of the diode region, and is excludedfrom peripheral portions of the diode region. The electrode 1330,preferably a transparent electrode, can be coextensive with, or smallerthan, the emission region 1320 a. Other techniques for reducing the areaof the emission region also may be used.

Stated differently, the substrate 1310 has first and second opposingfaces 1310 a and 1310 b, respectively. The first face 1310 a has smallersurface area than the second face. A diode region 1320 is on the secondface 1310 b. An emission region 1320 a is included in the diode region1320 and is confined to within the smaller surface area of the firstface 1310 a. This configuration can make the chip look more like a pointsource at a focal point of a lens. Conventionally, light generated atthe edges of the chip does not obtain much of a benefit of the shapededges, since a smaller solid surface of emitted light interacts withthose surfaces, compared to light generated in the center of the chip.Simulations show that about 20% more light output can be obtained bybringing the emission region in from the chip edges. This increasedextraction efficiency of a reduced emission area also can make the lightoutput less sensitive to losses in the substrate and at the substratesurface, since more light can escape on the first pass out of the activeregion. Embodiments of FIG. 13 may be used for conventionalnon-flip-chip packaging. Embodiments of ATON LEDs that may be used forflip-chip packaging will be described in connection with FIG. 17 below.

FIG. 14A illustrates application of a reduced emission area to otherembodiments of the invention. For example, LEDs 1400 are similar to LEDs700 of FIG. 7, except that at least one reduced emission area isprovided according to embodiments of the invention. At least one reducedemission area may be provided by one or more conductive electrodes 1410that are aligned with the pedestals 730. The conductive electrodes 1410may comprise platinum and/or other materials. An insulating layer 1420,for example comprising silicon nitride, may be used to prevent metalcontact to the diode region 740. An interconnect metal layer 1430 may beblanket formed on the conductive electrodes 1410 and on the siliconnitride layer 1420. It will be understood that the interconnect layer1430 also may act as a reflective layer. Alternatively, one or moreseparate reflective layers may be provided. FIG. 14B is a bottom view ofthe LEDs of FIG. 14A, illustrating the conductive electrodes 1410 andthe metal layer 1430. It also will be understood that in otherembodiments of the invention, at least one mesa 1320 a of FIG. 13 may beincluded in the diode region 740 of FIG. 14A, to provide at least onereduced emission area. Moreover, reduced emission areas may be used withother embodiments of the invention that are described herein.

Thus, efficiency of LEDs can be improved by reducing the emission areato be confined more in the central portions of the back-shaped regions.For a back-shaped device with a uniform emission area, much of theemission occurs over the shaped regions of the chip which may be lessefficient. In contrast, in FIGS. 14A and 14B, since the emission regionor regions are aligned to the pedestals 730, improved extractionefficiency may be provided. Since the conductive electrodes 1410 aredisconnected from each other, they may need to be connected with aninterconnecting conductive metal 1430. If the device is flip-chipbonded, the conductive metal 1430 can be the solder- and/orsilver-loaded epoxy. If the device is mounted in non-flip-chipconfiguration, a metal sheet such as a reflector, metal strips and/orwire bonds may be included for interconnection.

FIG. 15A illustrates application of at least one reduced emission areato yet other embodiments of the invention. For example, LEDs 1500 aresimilar to LEDs 800 of FIG. 8, except that at least one reduced emissionarea is provided according to embodiments of the present invention. Atleast one reduced emission area may be provided by a conductiveelectrode 1510 that does not overlap the via holes 820. The conductiveelectrode 1510 may comprise platinum and/or other materials. Aninsulating layer 1520, for example comprising silicon nitride, may beused to prevent metal contact over the areas defined by the via holes820. FIG. 15B is a bottom view of the LEDs of FIG. 15A, illustrating theconductive electrode 1510 and the insulating regions 1520. As was thecase with FIG. 14A, in other embodiments, at least one mesa 1320 a ofFIG. 13 may be included in the diode region 840 of FIG. 15A, to provideat least one reduced emission area.

It will be understood that embodiments according to the presentinvention can be used to improve sapphire-based nitride LEDs as well asLEDs based on other material systems, as was described above. However,in sapphire-based nitride LEDs, most of the light may remained trappedin the higher index nitride diode region. The gains may be greater forsilicon carbide, gallium nitride, aluminum nitride, zinc oxide and/orother substrates where the index of refraction of the substrate ishigher than that of sapphire. Thus, embodiments of the invention may notbe limited to the use of silicon carbide as a substrate. It will beunderstood that when sapphire is used as a substrate, the insulatinglayer of sapphire may use two contacts to the diode region, such as wasillustrated in FIG. 1. These contacts may need to be aligned to theirelectrical connections in the package. The transparency of a sapphiresubstrate can facilitate the alignment if the exposed side is polished.However, a polished exposed surface may be less efficient for extractinglight than a textured one.

FIG. 16 is a cross-sectional view of light emitting diodes according toother embodiments of the present invention is shown. FIG. 16 may beregarded as similar to FIG. 3, except the light emitting diode 1600 isconfigured for non-flip-chip mounting, rather than flip-chip mounting.Moreover, other contact structures between the LED and a mountingsupport, such as a mounting or sub-mounting assembly, also are shown. Itwill be understood that the non-flip-chip mounting of FIG. 16 and/or thecontact structures of FIG. 16 may be used with other embodiments of theinvention that are described herein, and/or with other conventionalLEDs.

More specifically, referring to FIG. 16, the light emitting diode 1600is mounted on a mounting support 210 in a non-flip-chip orientation,wherein the silicon carbide substrate 110 is adjacent the mountingsupport 210, and the diode region 170 is on the silicon carbidesubstrate 110 opposite or remote from the mounting support 210. Stateddifferently, the diode region 170 is up, and the silicon carbidesubstrate 110 is down. It has been found, according to embodiments ofthe invention that are illustrated in FIG. 16, that the amount of lightthat may be extracted from a conventional non-flip-chip mounted LED canbe increased when a transparent silicon carbide substrate 110 is used.In these embodiments, light from the active region 130 of the dioderegion 170 that is generated downward into the silicon carbide substrate110, is reflected back through the silicon carbide substrate 110 via areflector 140. Although the extraction efficiency may be less than LEDs300 of FIG. 3, extraction efficiencies greater than those ofconventional LEDs that do not employ transparent silicon carbidesubstrates 110, may be obtained. It will be understood that aright-side-up mounting of FIG. 16, with the transparent silicon carbidesubstrate adjacent a mounting region and the diode region remote fromthe mounting region, also may be used with other conventional LEDconfigurations.

Still referring to FIG. 16, contact structures 1620 according toembodiments of the invention now will be described. It will beunderstood that these contact structures 1620 may be used with otherembodiments that are described herein, and also may be used with otherconventional LED structures.

As shown in FIG. 16, the contact structure 1620 includes an ohmic region160, a reflector 140, a barrier region 1610, and a bonding region 230. Atop ohmic contact 150 also is provided. The ohmic contacts 150/160preferably are a transparent layer or layers of metal, such as a thinlayer of platinum, which may be between about 10 Å and about 100 Åthick. Other transparent ohmic contacts 150/160 may be used. Forexample, transparent oxides, such as indium tin oxide (ITO) may be used,in which case the thickness of the ohmic contact 150/160 may be up toseveral microns or more thick. The ohmic contact 150/160 can be auniformly thick layer, a grid structure and/or a dot structure, forexample as described in U.S. Pat. No. 5,917,202 to Haitz et al., thedisclosure of which is hereby incorporated herein by reference in itsentirety as if set forth fully herein. The ohmic contacts 150/160preferably provide current spreading, to facilitate efficient anduniform current injection into the active region 170. When a grid or dotstructure is used for the ohmic contacts 150/160, current spreading maybe achieved if the silicon carbide substrate 110 has properly matchedconductivity. Grid/dot ohmic regions can reduce or minimize the lightabsorption in the ohmic region due to reduced area coverage. It alsowill be understood that the configurations of the ohmic contacts 150 and160 need not be identical.

Still referring to FIG. 16, the reflector 140 can comprise, for example,between about 100 Å and about 5000 Å of reflecting metal, such as silverand/or aluminum, and/or a mirror stack as was already described. It alsowill be understood that the functionality of the ohmic region 160 andthe reflector 140 can be combined in a single ohmic and reflectorregion, such as a single layer of silver or aluminum, which canconcurrently provide a current spreading and a reflecting function.Alternatively, different layers may be used for the ohmic region 160 andthe reflector 140.

Still referring to FIG. 16, a barrier region 1610 also may be providedthat protects the reflector 140 and/or the ohmic region 160 fromdiffusion and/or spiking from an underlying die attach. Thus, thebarrier region 1610 can preserve the optical and/or electrical integrityof the ohmic region 160 and/or the reflector 140. In some embodiments,the barrier region includes between about 100 Å and about 5000 Å ofnickel, nickel/vanadium and/or titanium/tungsten.

Finally, still referring to FIG. 16, a bonding region 230 is providedthat adheres the semiconductor LED structure to a mounting support 210,such as a mounting or sub-mounting assembly. The bonding region 230 canbe a metal layer comprising, for example, gold, indium, solder, and/orbraze, and may include a preform of one or more of these and/or otherstructures. In some embodiments, the bonding region 230 can includesolder bumps and/or other metal bumps, such as indium or gold. Themounting support 210 may include a heat sink, a Surface Mount Technology(SMT) package, a printed circuit board, a driver integrated circuit, alead frame and/or other conventional mounting and/or sub-mountingassemblies that are used for LEDs. The bonding region 230 can be adheredto the mounting support 210, using silver epoxy, solder bonding,thermo-solder bonding and/or other techniques. It also will beunderstood that embodiments of contact structures 1620 of FIG. 16 may beused with flip-chip-mounted LEDs and/or other conventional LEDstructures.

FIG. 17A is a cross-sectional view of light emitting diodes according tostill other embodiments of the present invention. More specifically,FIG. 17A illustrates a conventional ATON LED that is flip-chip-mountedand/or employs a contact structure according to embodiments of thepresent invention. Flip-chip-mounting such as was shown, for example, inFIG. 2, and/or contact structures, such as were illustrated in FIG. 16,may be used.

Referring now to FIG. 17A, these embodiments of LEDs 1700 include atransparent substrate 1310 such as a colorless, compensated siliconcarbide substrate, and a diode region 1320 that is flip-chip-mounted ona mounting support 210, such that the diode region 1320 is adjacent themounting support 210, and the silicon carbide substrate 1310 is remotefrom the mounting support 210. The diode region 1320 may include areduced emission area such as a mesa area 1320 a of FIG. 13.

Flip-chip mounting, as shown in FIG. 17A, can provide improved lightextraction compared to conventional non-flip-chip mounted LEDs. Forexample, in conventional chips that use gallium nitride regions onsilicon carbide substrates, about half of the light that is generatedinternally in the LED active region may actually be emitted. Theremainder may be trapped in the semiconductor material, due to totalinternal reflection and/or absorption losses. A configuration whereinthe first face of the silicon carbide substrate, remote from the dioderegion, has smaller surface area than the second face of the siliconcarbide substrate, adjacent the diode region, can enhance lightextraction. However, a significant portion of the emitted light isincident and/or passes through chip faces that are completely and/orpartially covered with absorbing metal, to facilitate the ohmic contactto the LED p- and n-regions. Moreover, a conventional LED directs lightprimarily downward, which may cause an additional reflection fromoptical elements in the LED package, and/or loss due to the die attachmaterial. These phenomena may cause additional optical losses.

In sharp contrast, embodiments of the invention as illustrated in FIG.17A can employ an ATON geometry and/or other geometries that include areduced area first face 1310 a compared to the second face 1310 b, thatis flip-chip mounted to a mounting support 210, such that the dioderegion 1320 is adjacent the mounting support 210, and the substrate 1310is remote from the mounting support 210.

Moreover, as also shown in FIG. 17A, additional efficiency may beobtained by using p-contact structures 1740 that include a p-ohmicregion 1742, a reflector 1744, a barrier region 1746 and/or a bondingregion 1748. In some embodiments, the p-ohmic region 1742 may comprise ap-ohmic metal, such as nickel/gold, nickel oxide/gold, nickeloxide/platinum, titanium and/or titanium/gold, between about 10 Å andabout 100 Å thick. In some embodiments, the p-ohmic region 1742 cancomprise a continuous or non-continuous p-ohmic metal, with an areacoverage of between about 10% and about 100%, and a thickness of betweenabout 2 Å and about 100 Å. For non-continuous p-ohmic metals, theconductivity of the underlying diode's layers may be matched to the areacoverage, to enhance the uniformity of current injection into thediode-active region.

Still referring to FIG. 17A, a thick reflector 1744 such as silverand/or aluminum is on the ohmic region 1742, opposite the diode region1320. In other embodiments, the ohmic region 1742 can be thick enough tofacilitate a low contact resistance, but thin enough to reduce opticalabsorption. The ohmic region 150 may be equal to or less than half theohmic metal thickness of a conventional ATON chip, to accommodate thedouble pass of the light through the contact. Moreover, the currentspreading function can be assumed by a combined ohmic and reflectorregion, which can be designed to be thick enough to facilitate efficientoptical reflection of the light generated in the diode region 1320, andsimultaneously provide the current spreading. Thus, light 1726 that isgenerated in the diode region 1320 can reflect off the reflector 1742back into the substrate 1310. Other light 1722 generated in the dioderegion can be injected directly into the substrate 1310. Yet other light1724 can emerge from the oblique sidewalls of the substrate.

Still referring to FIG. 17A, the flip-chip mounted LED can be dieattached using a bonding region 1748 that can include gold, indium,conventional epoxy materials, braze and/or solder with the use ofappropriate solder and/or solder barrier regions 1746, such as nickel,nickel/vanadium and/or titanium tungsten. An optional adhesion layer,comprising, for example, titanium, also may be provided between barrierlayer 1746 and bonding region 1748. It also will be understood that, byproviding reflective metal on the semiconductor surface, a very smoothmirror surface may be achieved which has relatively high reflectivitycompared to the relatively rough surface of the header.

Still referring to FIG. 17A, an n-contact structure 1730 may be usedthat includes an ohmic/reflector region 1732, an adhesion region 1734, abarrier region 1736 and a bonding region 1738. The ohmic/reflectorregion 1732 may comprise an n-ohmic material, such as aluminum and/orsilver about 1000 Å thick. This ohmic/reflector region 1732 may act asan ohmic contact and as a reflector layer. An optional reflector regionalso may be provided that is similar to the reflector region 1724. Anoptional adhesion region 1734 comprising, for example, about 1000 Å oftitanium, may be provided. A barrier region 1736 also may be provided.For example, about 1000 Å of platinum may be used. Finally, a bondingregion 1738 may be used. For example, up to 1 μm or more of gold may beused, and a conventional wire bond 1750 may be attached to the bondingregion 1738. It also will be understood that one or more of the regions1732, 1734, 1736 or 1738 may be eliminated or combined with otherregions, depending upon the particular application.

Flip-chip mounted ATON chips according to embodiments of FIG. 17A canproduce about 1.5 to about 1.7 times or more enhanced radiometric fluxcompared to a conventional non-flip-chip ATON geometry. Absorption inthe p-electrode and/or die attach material can be reduced. An n-contactstructure 1730 may be used that can reduce or minimize the surfacecoverage. Thus, in some embodiments, an electrode structure 1730 canoccupy the entire first face 1310 a of the silicon carbide substrate1310. However, other geometrical configurations may be used to reduce orminimize the surface coverage.

For example, as shown in FIG. 17B, the n-contact structure 1730 mayinclude only a central portion 1730 a on the first face 1310 a. However,for larger chip sizes, fingers 1730 b of the n-contact structure 1730also may be employed to provide additional current spreading. Thus, then-contact structure 1730 may occupy the fall area of the first face1310. In other embodiments, less than the full area may be occupied, butmore than 10% of the area may be occupied. In still other embodiments,less than 10% of the area may be used. It also will be understood thatmany other geometric configurations of central portions 1730 a andfingers 1730 b may be provided.

To allow enhanced scalability, the n-contact structure 1730 can be aninterconnected grid structure that is matched in surface coverage andcurrent spreading resistant to the conductivity of the underlyingsemiconductor material 1310. By reducing and preferably minimizing thesurface coverage of the n-contact structures 1730 and employing silverand/or aluminum reflectors 1732, the absorption of the completedn-contact structures 1730 can be reduced and preferably minimized.

It will be understood that, in other embodiments, both the n- andp-electrodes may be formed on the side of the chip, for example, asdescribed in U.S. Pat. No. 5,952,681 to Chen. This may further reducethe light absorption in the ohmic contacts for the n-electrode.

It also will be understood that flip-chip mounting and/or contactstructures of FIGS. 17A and 17B may be used with other substrategeometries including cubic, triangular, pyramidal, truncated pyramidaland/or hemispherical, with reduced area first faces. Additional firstface patterning in the form of grooves, via holes and/or otherpatterning features also may be provided, as was already described.Finally, texturing and/or roughening, as was already described, also maybe employed. Roughening of the substrate and/or the diode region may beemployed. Thus, the reflector 1744 may be formed on an intentionallyroughened and/or patterned diode region 1320. This random roughness canenhance light extraction and/or decrease internal reflections. Aspecific pattern, such as a Fresnel lens, also may be used to direct thereflected light and/or enhance light extraction. The patterning can forman optical element once the reflector 1744 is applied. The dimension ofthe pattern may be on the order of a wavelength of the light that isemitted from the LED.

FIG. 18 is a cross-sectional view of light emitting diodes according tostill other embodiments of the present invention. More specifically,FIG. 18 illustrates a conventional ATON LED that includes a reflector onthe diode region thereof. It has been found, unexpectedly, according toembodiments of the invention, that by adding a reflector to the dioderegion of a conventional right-side-up ATON and/or other LED, anincrease in brightness of about 10% or more may be obtained compared toa conventional ATON and/or other LED that does not have a reflector onthe diode region at the top surface thereof. Thus, by at least partiallyblocking the top surface with a reflector, increased light emissionactually may be obtained.

More specifically, referring to FIG. 18, these LEDs 1800 include asubstrate 1310 and a diode region 1320 according to any of theembodiments described herein and/or according to conventional ATONand/or other LED configurations. An n-contact structure 1810 may beprovided, according to conventional ATON and/or other configurations.Alternatively, an n-contact structure 1810, comprising an adhesionregion 1812, such as about 1000 Å of titanium, a barrier region 1814,such as about 1000 Å of platinum and a bonding region 1816 such as about1 μm of gold, may be provided. The adhesion region 1812 also canfunction as an ohmic contact and may additionally be reflective.

Still referring to FIG. 18, a p-contact structure 1820 may be provided.The p-contact structure 1820 may include a transparent ohmic region 1830that can be similar to the ohmic layer 1742 of FIG. 17A. An adhesionlayer 1826, which can be similar to the adhesion layer 1734 of FIG. 17A,also may be included. A barrier layer 1824, which may be similar tobarrier layer 1736 of FIG. 17A, also may be included. A bonding region1822, which is similar to the bonding region 1738 of FIG. 17A, and awire 1840, which may be similar to the wire 1750 of FIG. 17A, also maybe included. Moreover, conventional ATON/ITP ohmic contact structuresalso may be provided. According to some embodiments of the presentinvention, the p-contact structure 1820 includes a reflector 1828therein, to reflect light back through the diode region 1320 and intothe substrate 1310. The reflector 1828 may be similar to the reflector1746 of FIG. 17A. As was described above, unexpectedly, by adding areflector 1828 to the top surface of the ATON and/or other LED,increased brightness, such as between about 1.2 and about 1.3 times ormore the brightness of a conventional ATON LED, may be obtained.

Referring now to FIG. 19, LED fabrication methods according toembodiments of the present invention now will be described. It will beunderstood that some of the blocks of FIG. 19 may occur in a differentorder from that which is shown, and some blocks may be performedsimultaneously rather than sequentially.

Referring now to FIG. 19 at Block 1910, a diode region is formed on asilicon carbide substrate. Preferably, a gallium nitride diode region isfabricated on a silicon carbide substrate, as described above. At Block1920, grooves are sawed, etched, laser cut, reactive ion etched, wetetched and/or otherwise cut, and/or reactive ion etching is performed toform via holes on the first face of the substrate, as was described inconnection with FIGS. 7-12 and 14-15. It will be understood that thegrooves and/or via holes may be formed at Block 1920 prior tofabricating the diode region on the substrate and/or after fabricatingthe diode region on the substrate.

Referring now to Block 1930, a contact structure is formed, for exampleas was described in connection with FIGS. 2-5, 11-12 and 16-17. It willbe understood that the contact structure may be formed prior to forminggrooves and/or reactive ion etching the via holes of Block 1920.

Referring now to Block 1940, dicing is performed to separate individualLED chips. It will be understood that dicing need not be performed if awafer size LED is being fabricated and dicing may be performed prior toforming the electrode structure of Block 1930 and/or prior to sawing thegrooves and/or reactive ion etching at Block 1920.

Then, referring to Block 1950, the diode is joined to a mountingsupport, for example using a solder preform and/or other joiningtechniques, for example as was described in connection with FIGS. 2-5,11-12 and 16-17. At Block 1960, the diode is then packaged, for examplein a plastic dome, as was illustrated in FIGS. 2-3 and 16. LEDs havinghigh extraction efficiency thereby can be manufactured efficiently.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

1. A light emitting diode comprising: a substrate; a diode region on thesubstrate comprising a gallium nitride based n-type layer, an activeregion and a gallium nitride based p-type layer; a first transparentoxide layer on the gallium nitride based p-type layer; a secondtransparent oxide layer on the substrate remote from the diode region; afirst reflector layer on the first transparent oxide layer remote fromthe gallium nitride-based p-type layer; and a second reflector layer onthe second transparent oxide layer remote from the substrate.
 2. A lightemitting diode according to claim 1 wherein the first transparent oxidelayer and the second transparent oxide layer both comprise Indium TinOxide (ITO).
 3. A light emitting diode according to claim 2 wherein theITO is several microns or more thick.
 4. A light emitting diodeaccording to claim 1 further comprising: a barrier layer on the secondreflector layer remote from the second transparent oxide layer; and abonding layer on the barrier layer remote from the second reflectorlayer.
 5. A light emitting diode according to claim 4 furthercomprising: a mounting substrate on the bonding layer remote from thebarrier layer.
 6. A light emitting diode according to claim 1 whereinthe substrate comprises silicon carbide.